Procerss for producing guanidine thiocyanate



1955 o. GROSSKINSK-Y ET AL K 2,700,056,

PROCESS FOR PRODUCING GUANIDINE THIOCYANATE Filed Aug. 5, 1952 M-\ R Q I iqwu/azxuw/ PEFJSVZE I l I l I I l l I -P (I I INVENTORSI 61-10 I G-RaSsKIMSKY AND HELMUT UMBACH United States Patent PROCESS FOR PRODUCING GUANIDINE THIOCYANATE Otto Grosskinsky and Helmut Umbach, Dortmund-Eving, Germany Application August 5, 1952, Serial N 0. 302,786

Claims priority, application Germany August 3, 1949 6 Claims. (Cl. 260-564) The present invention relates to the process of producing guanidine thiocyanate by a thermal conversion of ammonium thiocyanate.

The present application is a continuation-in-part of our co-pending U. S. application Serial No. 133,486, filed December 16, 1949, for Process of Making Guanidine Sulfocyanide. Application Serial No. 133,486 is now abandoned.

It is an object of the present invention to provide a process of producing guanidinc thiocyanate by the then mal conversion of ammonium thiocyanate, which process results in highly increased yields of guanidine thiocyanate.

It is another object of the present invention to provide a process of converting ammonium thiocyanate into guanidine thiocyanate by the utilization of considerably higher temperatures than could hitherto be employed.

It is still another object of the present invention to provide a process of converting ammonium thio'cyanate into guanidine thiocyanate much more rapidly and in greater yield than by previous known processes without the danger of producing undesirable side reactions.

With the above objects in view, the present invention mainly comprises heating ammonium thiocyanate in a closed reaction vessel to the melting point of the ammonium cyanate so as to convert a portion of said ammonium thiocyanate into guanidine thiocyanate and gaseous hydrogen sulfide, thereby raising the pressure to a superatmospheric pressure, said superatmospheric pressure preventing side reactions and consequent formation of undesired side products, the superatmospheric pressure being predetermined for equilibrium conditions; continuously raising the temperature to a maximum of 300 C. while maintaining said predetermined 'superatmos'pheric pressure by purging excess hydrogen sulfide above the amount necessary for said 'superatmospheric pressure in said reaction vessel, thereby preventing reaction of said hydrogen sulfide with the still unconyerted ammonium thiocyanate and thus permitting conversion of substantially all of said ammonium thiocyanate into gnanidine thiocyanate; and slowly cooling the reaction vessel and the contents thereof while removing the remainder of the formed hydrogen sulfide at a speed such that substantially all of said hydrogen sulfideis removed at about the time the temperature is reduced to the desired temperature which is generally below 210 C. so that the pressure in said reaction vessel is reduced to substantially atmospheric pressure at about the time the desired. cooling temperature is reached, thereby terminating thereactions and obtaining guanidine thiocyanate.

Preferably, the predetermined temperature is chosen between 210300 (3., the reaction proceeding best'and most rapidly within this temperature range. The maximum temperature obtainable depends upon the speed at which the ammonium thiocyanate can be heated to this temperature as shall be more clearly explained. The superatmospheric pressure utilized depends, to some extent, on the reaction temperature and is preferably maintained between -50 atmospheres and most preferably between 2530 atmospheres. Reasons for these preferences will be apparent from the discussion which follows:

The conversion of ammonium thiocyanate into guanidine thiocyanate proceeds substantially according to the following equation:

(1) NH2.HSCN

2,700,056 Patented Jan. 18, 1955 If the above reaction were the only one which took place, it would be obvious to remove the hydrogen sulfide evolved as rapidly and completely as possible, in order to push the equilibrium to the right. Since the hydrogen sulfide formed is gaseous, this could be simply attained by working in an open vessel at atmospheric pressure. However, there are a number of side reactions which prevent this apparently simple production of guanidine thiocyanate in good yield.

Assuming a complete course of the above reaction, 128.83 parts of ammonium thiocyanate would yield parts of guanidine thiocyanate. However, due to the law of mass action and because of the side reactions involved, the best yield hitherto obtainable in the above process, even utilizing a reaction time of 20-30 hours, was about 60% of the theoretical yield, i. e., in lieu of 100 parts of gnanildine thiocyanate only about 60 parts could be obtame Moreover, side reactions are involved which complicate the process and make the .attainmentof even a 60% yield very difficult. The most important of these side reactions is the following:

The hydrogen sulfide, reacts with the still unreacted ammonium thiocyanate to produce carbon disulfide and ammonia, thereby considerably lowering the yield of guanldine thiocyanate. When working in an open vessel, carbon disulfide and ammonia will escape together with the hydrogen sulfide, thus involving considerable losses of ammonium thiocyanate which of course lowers the yield of guanidine thiocyanate. This is' one of the major reasons for obtaining only poor yields when car rying out the thermal conversion of ammonium thiocyanate into guanidine thiocyanate'in an open vessel.

It has been proposed to conduct a current of gaseous ammonia through'the molten mass subjected to thermal treatment so as to force the equilibrium of the above reaction to the left. However, this method is complicated because of the need for an additional ammonia source and the need for an apparatus for recovering the ammonia employed. A further disadvantage of this method is the fact that the ammonia causes an additional side reaction which produces considerable amounts of undesired ammonium trithiocarbonate which causes the danger of stoppage in the delivery pipe of the reaction vessel.

It has also been proposed that by mere thermal treatment of ammonium thiocyanate, i. e., a treatment without adding ammonia or other reagents, the yield might be improved by carrying out the treatment in a closed vessel and at superatmospheric pressure. However, although superatmospheric pressure pushes the equilibrium of Equation 2 to the left, it also pushes the equilibrium of Equation 1 to the left, which is, of course, disadvantageous.

The process of the present invention eliminates the difliculty of operating in an open vessel and also of operating in a closed vessel under superatmospheric pressure by providing for the blowing olf of hydrogen sulfide from the reaction vessel when the equilibrium has been attained. This blowing off is effected slowly and carefully so that the pressure in the reaction vessel drops very slowly. By proceeding in this manner the reaction according to (l) is set going again while at'the same time the undesired reaction (2) is effectively hampered by the removal of the hydrogen sulfide and the prevailing superatmospheric pressure.

The deliberation of the hydrogen sulfide gradually becomes lessened so that the equilibrium pressure slowly drops. The end of the conversion may be recognized by the fact that when closing the blowing-off valve on the reaction vessel, no further rising of pressure is observed because hydrogen sulfide is no longer being evolved. The heating is then stopped and the vessel allowed to cool to about 210 C. During this cooling the residual hydrogen sulfide is allowed to blow off at such rate that when the desired cooling temperature has been reached the pressure is approximately atmospheric.

It is known that melamine is formed when ammonium thiocyanate is subjected to high temperatures, this reaction having been made use of in a commercial production of melamine by heating ammonium thiocyanate to about 370380 C. in a closed vessel. It was therefore always considered that by using very high temperatures when heating ammonium thiocyanate, a reaction product would be obtained consisting essentially of, or at least contaminated by melamine. For this reason all known processes for the production of guanidine thioeyanate by the thermal conversion of ammonium thiocyanate used temperatures between l80200 C., the reaction starting at about 170 C. No higher temperatures than 200 C. were employed because of the danger of contamination of the reaction product with melamine.

The present invention can employ temperatures well above 210 C., and as high as 300 C. or higher, the process resulting in improved yields of guanidine thiocyanate, a considerable reduction in the time of reaction and the elimination of undesirable side products.

It has been found by the present invention that it is extraordinarily advantageous in the production of guanidine thioeyanate by a thermal conversion of ammonium thiocyanate to compromise between the two known processes in a very specific manner which forms the basis of the present invention. The compromise is between the process of heating ammonium thiocyanate at atmospheric pressure in an open vessel and heating ammonium thiocyanate at superatmospheric pressure in a permanently closed vessel. The compromise consists in first carrying out the heating of the ammonium thiocyanate at superatmospheric pressure and allowing the hydrogen sulfide evolved to blow off, either continuously or periodically.

Due to the small amounts of guanidine thiocyanate present at the beginning of the reaction, the conversion according to Equation 1 takes place very briskly thus allowing for keeping the hydrogen sulfide in the system (the presence of hydrogen sulfide theoretically handicapping the conversion). Accordingly, the conversion is carried out under a certain superatmospheric pressure caused by the evolved hydrogen sulfide, this manner of proceeding offering the advantage that the reaction according to Equation 2 courses substantially from the right to the left, so that the carbon disulfide and the ammonia evolved react to form ammonium thiocyanate, the latter being subsequently converted into guanidine thiocyanatc and hydrogen sulfide according to Equation 1.

However, when working in this manner, it would be a mistake notto remove the hydrogen sulfide at all. On the contrary, the hydrogen sulfide is allowed to slowly blow off either continuously or periodically when the equilibrium pressure, depending on the predetermined temperature to some extent, is attained. In this manner of working, there is another matter which must also be taken into consideration. Heating the charge in a closed vessel to a distinct temperature and waiting until the actual final equilibrium has been established would be time wasted because the pressure rises asymptotically to its maximum value. It is of course possible to carry out the process in this manner by waiting for the equilibrium pressure to be reached before the hydrogen sulfide is allowed to blow off.

In practice however it is desirable to somewhat modify the process so as to reduce the operating time perceptibly. According to this modification the heating is carried out in a closed vessel but the blowing off of the hydrogen sulfide is started before the real equilibrium pressure and the final operating temperature has been obtained. Durmg the blowing off of the hydrogen sulfide the temperature is raised to the final operating temperature, the blowing off of the hydrogen sulfide during this period being controlled in such manner that no essential reduction of the pressure occurs. When the maximum temperature has been obtained the pressure gradually drops while the blowing oif of the hydrogen sulfide is continued and the process is continued while further blowing off hydrogen sulfide, the pressure gradually dropping, until the conversion is practically complete.

Both manners of operation, namely keeping the vessel closed until the actual equilibrium pressure is reached, and the method of allowing the hydrogen sulfide to blow off slowly before the actual equilibrium pressure is reached, while continuing to heat and without allowing the pressure to drop perceptibly, are very satisfactory and give yields of guanidine thiocyanate of 90% and more of the theoretical value, the amount of by-products such as carbon disulfide and ammonia being nominal. When carrying out the process in an open vessel, about 19-20% by weight of the charge is lost as carbon disulfide, whereas this loss is reduced to about 0.70.8% by Weight by the present invention.

The reason for proceeding according to the embodiment of allowing the hydrogen sulfide to escape before the actual equilibrium pressure is achieved may be best understood from the following description of the attached graph.

This graph points out the asymptotic rise in pressure and the reason for proceeding as described. The reaction time is represented as abscissa, the pressure indicated as ordinate. The graph shows that it would be of little use to wait until the pressure has risen from value p to the final equilibrium pressure, since the value between the two pressures is nominal. The present invention aims mainlyat carrying out the entire conversion as rapidly as possible in order to avoid waste of time which increases the risk of formation of by-products such as melamine.

As soon as the temperature at which the reaction starts, about 170 C., is attained, it is preferable to carry out the heating to the final operating temperature as rapidly as possible. The more rapidly the heating to the operation temperature is effected, the more rapidly the reaction proceeds and the better the yield of guanidine thiocyanate, since at a slow course of reaction the decomposition of the guanidine thiocyanate which was formed becomes perceptibly important.

The rapid elevation of the temperature involves the additional advantage that the more rapid the elevation of the temperature, the higher the maximum operating temperature and the more rapid the course of the conversion. In other words: By a rapid elevation of the temperature a higher maximum temperature is attainable than by a slow elevation of the temperature. Therefore, the most favorable maximum temperature per se cannot be definitely stated since such temperature depends on the rate of raising the temperature thereto.

According to the present invention temperatures are appliable, according to the rate of heating up, which are far above the limit of 200 C. utilized hitherto, this being attained by rapid heating, as stated above. If the heating up to 200 C. takes 2 hours, for example, the process may be finished at about 220 C., the entire conversion process requiring about 3 hours and a half. When heating up more rapidly still, say, to 200 C. within half an hour, a maximum temperature of about 270 C. may be attained. In this case the conversion necessitates only one and one-half hours.

The temperature of conversion in the reaction vessel may even be raised up to 300 C., such temperature involving a reaction time correspondingly reduced, without the risk of troubles due to decomposition of the charge or formation of by-products, especially melamine and related compounds, this being based on the high pressure involved by the correspondingly high temperature.

The amount of hydrogen sulfide evolved being considerable, i. e. parts NH4SCN will yield 22 parts HzS, the hydrogen sulfide violently evolved must be allowed to blow off either continuously or periodically when the operation pressure is attained.

The superatmospheric pressure employed also suppresses the formation of melamine, melam, melon, and other undesired by-products. It is therefore desirable to use as high a pressure as possible, depending only on the resistance of the autoclave. Pressures of 50 atmospheres and more may therefore be employed, and throughout it is possible to keep the vessel closed until the chemical equilibrium in said vessel is attained, according to the temperature chosen. Nevertheless, the use of extremely high temperatures involves high apparatus expenses; therefore pressures not exceeding 50 atmospheres are most preferable, though eventually such pressures may be still below the real equilibrium pressure attainable. Principally, pressures as high as possible are desirable since, the higher the pressure, the more efiiciently the formation of undesired by-products will be suppressed.

As soon as the chemical equilibrium has been attained or the formation of guanidine thiocyanate perceptibly drops, the conversion according to Equation 1 is set going again, this being effectuated by allowing the hydrogen sulfide evolved to slowly blow off, either continuously or periodically. After a time, the pressure begins to drop, this dropping being due to the fact that the rate at which the hydrogen sulfide is evolved gradually decreases. Subsequently the pressure is gradually reduced by allowing further mo of hydrogen sulfide to slowly blow ott- 'rr s ea es t e suitabl m Lent o stopp n the conversion. Theoretically, the process should be stopped at the latest when in the reaction mixture as "much guanidine thiocyanate is decomposed to form. melamin ke products as guanidine thiocyanate is formed of ammonium thiocyanate and thiourea, respectively.

When the conversion has been finished, the heat supply is stopped and the reaction vessel allowed to cool down to a discharging temperature, said temperature being below the temperature (about 210 C.) at which the formation of by-products proceeds slowly enough as to be of practically no importance, and above the melting temperature of the reaction product (about 120 C.); Generally a cooling down to a temperature of 2052l0 C. will be sufficient. During saidcooling stage the residual hydrogen sulfide is allowed to slowly blow ofi in such a manner that when the discharging temperature has been attained, the pressure in the reaction vessel has substantially dropped to atmospheric pressure.

The subsidence of the rate of conversion of ammonium thiocyanate to guanidine thiocyanate, as well as that of the formation of hydrogen sulfide is only a criterion for an opportune stopping of the process. In carrying out the process it is therefore advisable to proceed by first allowing the hydrogen sulfide pressure prevailing above the molten mass to rise, and to start to reduce the pressure no sooner than the time at which thereaction commences to relax according to the mass equilibrium of the Equation 1. The hydrogen sulfide is subsequently allowed to blow off continuously or periodically under constant or varying pressure, a great part of the ammonium thiocyanate having been at that time converted to guanidine thiocyanate.

The process may be effected continuously by passing the ammonium thiocyanate through a heated pressureresistant vessel, i. e., through a vertical pressure-resistant tube lined with enamel. The evolved hydrogen sulfide is suspended as a foam in the molten mass and allowed to blow off at the top of the vessel. A slightly inclined tube may be employed also at one end the ammonium thiocyanate is fed in and at the other end the guanidine thiocyanate formed and the hydrogen sulfide evolved are drawn off. It need be, the dropping of the reaction may take place in a second stage apparatus so that practically all of the ammonium thiocyanate is completely converted into guanidine thiocyanate.

According to Equation 2, the equilibrium of the respective reaction is obviously displaced to the left side of said equation. as desirable, by increasing the concentration of CS2 and NHs, this is capable of being effectuated by forcing additional CS2 and NH3 into the reaction vessel. In this way, the small amounts of carbon disulfide and ammonia obtained as by-products may be reutilized in a suitable manner.

Summing up, the gist of the present invention consists in a combination of the following characteristics:

(1) Melting the ammonium thiocyanate by heating same to a temperature substantially not surpassing the melting point (150 (3.);

(2) Rapidly heating the molten ammonium thiocyanate in a closed vessel to a distinct maximum temperature chosen between 190 and 300 C.;

(3) Keeping said maximum temperature chosen until the chemical equilibrium has been attained, a distinct pressure being involved hereby in the closed vessel;

(4) Keeping said maximum temperature and allowing the hydrogen sulfide evolved to slowly blow off whereby the pressure is gradually reduced, and continuing said blowing otf at said temperature until the conversion of ammonium thiocyanate into guanidine thiocyanate is practically terminated;

(5) Stopping the heat supply, cooling the reaction product in the vessel to a discharging temperature ranging between a temperature at which practically side reactions do no longer occur (said temperature being about 210 C. and below) and the melting point of the reaction product (about 120 C.), and allowing the residual hydrogen sulfide to blow off at such a rate that when the discharging temperature has been attained, the pressure in the reaction vessel has dropped substantially to that of the atmosphere;

(6) Removing the liquid reaction product from the vessel.

.In l eu o work ng g fin ng o (3). Pr e a y the l'fwin modification may-beapplied:

(3 allowing the hydrogen sulfide evolved to slowly blow off, raising the temperature toa maximum value not surpassing about 300 C. during said blowing ofif period, the raising of the temperature and the blowing off of H28 being so balanced as to "keep the pressure substantially constant until the maximum 'value of the temperature has been attained.

The following example of the process of the present invention clearly indicates the extraordinarily short reaction time which could not hitherto be employed for the thermal conversion of ammonium thiocyanate into guanidine thiocyanate, the process further being remarkable because of the comparatively rigorous conditions of temperature without resulting in the formation of any perceptible amounts of melamine or melamine-like byproducts. It is to be understood, however, that the following is exemplato'ry only and that the scope of the present invention is not meant to be limited thereto. All parts are given by weight.

Example parts of ammonium thiocyanate is melted in an enamel-lined autoclave. The temperature of the charge israised above the melting point of the ammonium thiocyanate. The formation of guanidine thiocyanate commences at about C., as may be observed by the rise of pressure due to the formation of hydrogen sulfide.

After 60 minutes, counted from the end of the time the ammonium thiocyanate melts, the temperature rises to C. and the pressure to 25 atmospheres. While further raising the temperature by the ap'plication of external heat, the pressure is kept constant at 25 atmospheres by allowing the hydrogen sulfide to continuously blow off. After another heating period of 60-90 minutes, a temperature of 230 C. is obtained and the evolvement of hydrogen sulfide begins to slow on spite of further raising the temperature.

This point having been attained, the temperature is gradually dropped to about 210 C. and the pressure dropped simultaneously to atmospheric pressure within a further period of 30-60 minutes. The entire operation requires only about 3 hours. 78.8 parts of a crude molten mass is obtained, said mass containing 88.5% guanidine thiocyanate, 5.3% ammonium thiocyanate and 3.8% thiourea. 20.2 parts hydrogen sulfide, 0.3 part carbon disulfide and 0.5 part ammonia are obtained as by-products. The yield of guanidine thiocyanate is 90% of the theoretical value. Recrystallization from water results in a product containing 99100% pure guanidine thiocyanate. The ammonium thiocyanate (5.3%) and the thiourea (3.8%) contained in the crude molten mass may be readily recovered and returned to the process without being subjected to a preliminary purification.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptions should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. A process of producing guanidine thiocyanate, comprising the steps of heating ammonium thiocyanate in a closed vessel to about the melting point of said ammonium thiocyanate until a predetermined super-atmospheric pressure corresponding to a predetermined equilibrium level is reached due to the formation of gaseous products in said vessel while the same remains closed; and constantly increasing the temperature to a maximum of 300 C. while maintaining said predetermined superatmospheric pressure constant at said equilibrium level by purging the excess hydrogen sulfide over the amount necessary for said predetermined superatmosphereic pressure, until substantially complete conversion of said ammonium thiocyanate into guanidine thiocyanate.

2. A process of producing guanidine thiocyanate, comprising the steps of heating ammonium thiocyanate in a closed vessel to about the melting point of said ammonium thiocyanate until a predetermined superatmospheric pressure corresponding to a predetermined equilibrium level is reached due to the formation of gaseous products in said vessel while the same remains closed; and constantly increasing the temperature to a temperature between 230280 C. while maintaining said predetermined superatmospheric pressure constant at said equilibrium level by purging the excess hydrogen sulfide over the amount necessary for said predetermined superatmospheric pressure, until substantially complete conversion of said ammonium thiocyanate into guanidine thiocyanate.

3. A process of producing guanidine thiocyanate, comprising the steps of heating ammonium thiocyanate in a closed vessel to about the melting point of said ammonium thiocyanate until a predetermined superatmospheric pressure between 5-50 atmospheres corresponding to a predetermined equilibrium level is reached due to the formation of gaseous products in said vessel while the same remains closed; and constantly increasing the temperature to a maximum of 300 C. while maintaining said predetermined superatmospheic pressure constant at said equilibrium level by purging the excess hydrogen sulfide over the amount necessary for said predetermined super.- atmospheric pressure, until substantially complete conversion of said ammonium thiocyanate into guanidine thiocyanate.

4. A process of producing guanidine thiocyanate, comprising the steps of heating ammonium thiocyanate in a closed vessel to about the melting point of said ammonium thiocyanate until a predetermined superatmospheric pressure between 5-50 atmospheres corresponding to a predetermined equilibrium level is reached due to the formation of gaseous products in said vessel while the same remains closed; and constantly increasing the temperature to a temperature between 230-280 C. while maintaining said predetermined superatmospheric pres sure constant at said equilibrium level by purging the excess hydrogen sulfide over the amount necessary for said predetermined superatmospheric pressure, until substantially complete conversion of said ammonium thiocyanate into guanidine thiocyanate.

5. A process of producing guanidine thiocyanate, comprising the steps of heating ammonium thiocyanate in a closed vessel to about the melting point of said ammonium thiocyanate until a predetermined superatmospheric pressure corresponding to a predetermined equilibrium level is reached due to the formation of gaseous products in said vessel while the same remains closed;

constantly increasing the temperature to a maximum of 300 C. while maintaining said predetermined superatmospheric pressure constant at said equilibrium level by purging the excess hydrogen sulfide over the amount necessary for said predetermined superatmospheric pressure, until substantially complete conversion of said ammonium thiocyanate into guanidine thiocyanate; and cooling the contents of said vessel to a temperature below about 210" C. while purging the formed hydrogen sulfide until atmospheric pressure is reached in said reaction vessel, the speed of said purging being adjusted so that atmospheric pressure is reached at about the time the temperature reaches 210 C.

6. A process of producing guanidine thiocyanate, comprising the steps of heating ammonium thiocyanate in a closed vessel to about the melting point of said ammonium thiocyanate until a predetermined superatmospheric pressure corresponding to a predetermined equilibrium level is reached due to the formation of gaseous products in said vessel while the same remains closed; constantly increasing the temperature to a maximum of 300 C. while maintaining said predetermined superatmospheric pressure constant at said equilibrium level by purging the excess hydrogen sulfide over the amount necessary for said predetermined superatmospheric pressure, until substantially complete conversion of said ammonium thiocyanate into guanidine thiocyanate; adding ammonium thiocyanate to said vessel while maintaining said superatmospheric pressure and said temperature; and maintaining said superatmospheric pressure by purging excess hydrogen sulfide while continuing to heat to a maximum temperature of 300 C. until substantially all of said ammonium thiocyanate is converted into guanidine thiocyanate.

References Cited in the file of this patent UNITED STATES PATENTS 1,902,400 Gluud Mar. 21, 1933 2,524,054 Hill Oct. 3, 1950 FOREIGN PATENTS 800,663 Germany Nov. 27, 1950 OTHER REFERENCES 13glrallz J. Chem. Soc., vol. 103 (1913), pp. 1378 to 

1. A PROCESS OF PRODUCING GUANIDINE THIOCYANATE, COMPRISING THE STEPS OF HEATING AMMONIUM THIOCYANATE IN A CLOSED VESSEL TO ABOUT THE MELTING POINT OF SAID EMMONIUM THIOCYANATE UNTIL A PREDETERMINED SUPER-ATMOSPHERIC PRESSURE CORRESPONDING TO A PREDETERMINED EQUILIBRIUM LEVEL IS REACHED DUE TO THE FORMATION OF GASEOUS PRODUCTS IN SAID VESSEL WHILE THE SAME REMAINS CLOSED; AND CONSTANTLY INCREASING THE TEMPERATURE TO A MAXIMUM OF 300* C. WHILE MAINTAINING SAID PREDETERMINED SUPERATMOSPHERIC PRESSURE CONSTANT AT SAID EQUILIBRIUM LEVEL BY PURGING THE EXCESS HYDROGEN SULFIDE OVER THE AMOUNT NECESSARY FOR SAID PREDETERMINED SUPERATMOSPHERIC PRESSURE, UNTIL SUBSTANTIALLY COMPLETE CONVERSION OF SAID AMMONIUM THICYANATE INTO GUANIDINE THIOCYANATE. 